Here is a sample session. We use $ for the Unix shell prompt, and # for the Ocaml shell prompt.
$ rlwrap swipl
130f@ieng6-202]:~:501$ swipl
Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 5.10.5)
Copyright (c) 1990-2011 University of Amsterdam, VU Amsterdam
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.
For help, use ?- help(Topic). or ?- apropos(Word).
?- X = cat.
X = cat.
?- X = Y.
X = Y.
?- halt.